Scope question/problem

[Speakerguy]

OK, so I got my new DPO3000 scope.

On DC and AC coupling, full bandwidth mode, probes shorted tip to ground clip, I get 20mv to 40mv pk-pk sinusoidal noise around 80-100MHz, all channels, all probes. It seems to vary as I move a probe around, as if acting like an antenna. If I GND couple a channel, it totally disappears, absolutely no noise.

We do have a problem with a nearby radio station or other broadcast that really messes up our channel 12 reception on cable TV (USA, NTSC). We have some regular coax feeds to some TV's and those suck, and then we have some super shielded coax in the newer part of the house and those TV's are better but still very noise picture on channel 12.

Could my channel 12 problem be the same thing the scope is seeing? I was disappointed to see this much noise on all channels, all probes unless I am GND coupled (and then it totally disappears). I might have to trek across town and plug it in and do the same test.

Any help is appreciated!

 

[Speakerguy]

Also, I live about two blocks from this thing:

Tower

I don't know what station it is, but I am looking into it.

 

[dknguyen]

Or you could fire it up inside a steel shed and see what happens.

 

[crutschow]

Sounds like you may be picking up an FM station which operate in the 88MHz to 108MHz region. The scope probe is apparently acting as an antenna. Do you see a signal with no probe connected to the scope?

Try connecting a length of plain wire to the scope input. Does the scope signal increase? If so that also indicates it's being picked up as an antenna.

Channel 12 is at 205MHz but you could be seeing a harmonic of the 100MHz signal in you TV.

The only legal ways to get rid of the interference are to wrap your house in a faraday shield or move. Cutting the cable to the tower is frowned on by the legal authorities.

 

[Speakerguy]

My bad, ignore previous 80-100MHz comment. I was counting roughly two cycles per time division and thought I had it on 20ns/div but it was actually 10ns/div. The memory storage length was also set too long so the DPO part wasn't looking quite like an analog scope would (sort of persistence-y). The persistence-y look helps see the sinusoidal nature.

So instead of guessing I actually turned on the frequency measurement and knocked the mem length down to 10kpts. The frequency measurement bounces around a little but sure enough it centered somewhere just above 200MHz - CATV channel 12 is 204MHz and that's our noisy channel.

There is some broadband noise when the input is turned on but no probe connected.

Please see attached screen shot for what is going on... I had to zip it because the board won't accept TIF files. CH1 is GND coupled shorted probe, 2 is no probe connected DC coupled, 3 is AC coupled shorted probe, 4 is DC coupled shorted probe. Triggering on CH3 but 4 is pretty plainly the same signal.

 

[Speakerguy]

Confirmed on a local web forum that the transmitting tower two blocks from me is broadcasting Channel 12.

Also, if you have trouble opening the TIFF, try using Microsoft Office Picture Manager.

So is lead paint still available?

 

[Mikebits]

Won't the end of Analog TV at the end of the year solve your problem?

 

[crutschow]

We do have a problem with a nearby radio station or other broadcast that really messes up our channel 12 reception on cable TV (USA, NTSC). We have some regular coax feeds to some TV's and those suck, and then we have some super shielded coax in the newer part of the house and those TV's are better but still very noise picture on channel 12.

The problem is that the noise is getting into the old coax which then contaminates the signal in the new coax, even though it's better shielded. To improve things you would likely need to run the super shielded coax for all the lines. Running the coax in grounded metal conduit or grounded metal braid would be ideal, but probably not practical.

 

[crutschow]

Won't the end of Analog TV at the end of the year solve your problem?

Cable companies may continue to transmit analog past the deadline for killing analog over-the-air broadcasts. They are under no mandate to stop.

To convert to digital he would, of course, have to buy digital TV's or rent a digital cable box(s).

 

[mneary]

Cable companies may continue to transmit analog past the deadline

Cable companies are not allowed to transmit. Their signals must be strictly confined to their own cables. Anything that would influence a scope would be way way above the limits.

 

[Mikebits]

Sounds like a mandate to me.
Senate approves 2009 for end of analog TV - MSNBC Wire Services - MSNBC.com

 

[Speakerguy]

Back to the scope thing...

I went to my Dad's office building tonight. In the hallway on the bottom floor of a 4 story building I plugged into a wall outlet. All signs of the 200MHz sinusoidal waveform were gone. The only thing left was 12.2mv pk-pk broadband noise, which still seems high to me but this building has flourescent lights, etc all over the place. GND coupling the channel made it totally clean. 12.2mv seems a little high to me, what about you guys? I also have a little DC offset (about 3mv) but I think that can be corrected for with the scope. All this work was done on 10mv/div scale.

 

[mneary]

Any scope probe will pick up stuff out of the air. "Grounding" the scope probe only eliminates the signal under some circumstances. It often just creates a pickup loop. In your case, 204 MHz has a 1/4 wavelength of about 30 cm, which is probably shorter than the length of your probe cable.

This is your (unwanted?) opportunity to learn more about RF. Myself, I find it fascinating. You can experiment with loops that pick up the 204 MHz even better. This sounds silly, but in the end you'll learn how to suppress it, too.

Suppressing unwanted pickup from an external field generally involves minimizing the total loop area between 'signal' and 'ground' (neither of which are absolutes).

Although analog broadcasting is supposed to cease in February 2009 (USA) the channels will eventually be sold to others who will replace the current QRM with their own.

Cable TV in the US is not required to stop distributing analog signals in 2009. It's their cable and they can use the bandwidth as they wish. They cannot broadcast analog signals, but this is no different. They were never allowed to broadcast anything. There are strict rules requiring them to confine the signals to their cable.

Lead paint isn't electrically conductive.

 

[Mikebits]

What your talking about is what we use to call a sniffer probe.

Lead paint isn't electrically conductive.

Back when I use to be in the Navy, someone had painted one of our Radar antennas with lead base paint. After many blown magnetrons, a Raytheon Engineer figured out that the antenna was covered with lead paint.

Back to your regularly broadcasted post

 

[mneary]

a sniffer probe

We still call it the same.

figured out that the antenna was covered with lead paint.

It might work for radar (GHz) but I doubt it would make a difference at 204 MHz.

 

[Sceadwian]

Lead based paint usually isn't electrically conductive, it is however electro-magnetically conductive. Even electrically isolated the sheer amount of conductive molecular elements spread across space are equivalent to an RF short circuit. As frequency increases the dielectric becomes less and less an issue.

 

[kchriste]

This might be the time to build a Faraday workshop! Just kidding as it is overkill for hobby use.
But it is what I use at work when repairing sensitive instruments. I'd go crazy chasing ghosts otherwise. See if your scope probes came with some little spring like pieces which fit over the ground ring on the tip. You'll have to pull off the hook clip to see the ground ring near the probe tip. These are used to provide a shorter ground path when working with RF circuits.

 

[Speakerguy]

Yeah, I've used those before, we called them 'pigtails' at the last place I worked (doing PWM amplifier stuff and having to look at edges). I'll see if the probes came with any.

Interesting fact, I looped the wire around itself several times and by varying the orientation was able to make the noise go from >60mv pk-pk to 18mv pk-pk.

I still want to try it in other locations to make sure there's not some issue with the scope and noise, it being a rather expensive purchase for me. But I am feeling a little bit relieved. Still, 12mv pk-pk at Dad's office building seemed like a little more than I expected.

If anyone can tell, I'm real concerned about noise in the work that I do

 

[Mikebits]

I imagine that the noise level has such a low current, that when the probe is actually on a circuit under test, the signal will vanish.

 

[Speakerguy]

I had the ground clip on the probe tip though...